This research application proposes a study of HIV and other infectious diseases that spread among intravenous drug users via shared injection equipment. The novel aspect of this study is to "let the needles do the talking" via a new data collection device that tracks used needles, and tests them for HIV infection. Such a Syringe Tracking and Testing System (STT) has already been implemented in New Haven; thousands of needles have been tracked, and hundreds tested for HIV using polymerase. chain reaction (PCR). The specific aims of this project are to: expand the existing STT to encompass rigorous data management and modeling; establish PCR protocols for reliable determination of HIV prevalence in needles, and broaden PCR testing to include other infectious diseases transmitted by needle sharing; develop statistical methods for estimating the prevalence and incidence of infectious diseases among intravenous drug users using STT data; develop mathematical modeling techniques for studying the transmission dynamics of 7-infectious diseases among intravenous drug users using STT data; and use STT data to examine the validity of self-reported sharing rates among intravenous drug users.